subject: Insulator is the "heart" of the isolation barrier - Digital Isolators [print this page] Insulator is the "heart" of the isolation barrier - Digital Isolators
Unlike the optocoupler, each ISOpro channel occupies little die area, making possible costeffective, high-channel-count isolators. Additionally, monolithic semiconductor process technology enables ISOpro technology to be combined with other semiconductor functions and processes to create highly integrated products with built-in isolation, such as isolated analog data converters and communication transceivers. These integration advantages enable a broader, more comprehensive ISOpro product roadmap than the optocoupler.
System Considerations
Regardless of implementation, isolators must meet safety standards for robust galvanic isolation. They must also be reliable enough to outlast the equipment in which they are installed, which, in the industrial world, can mean decades of use. Designers must ensure that isolation circuits can withstand electrical stresses that can cause physical damage and reject data-corrupting noise from any number of sources. Therefore, the designer must carefully consider key isolator operating parameters, such as common-mode transient immunity, key timing parameters, such as propagation delay and pulse width distortion, and field-related specifications, such as EMI and RF susceptibility. Likewise, continuous working voltage and mean time-to-failure (MTTF) are important indicators of device service life.
High Voltage Insulation Reliability
Insulation reliability directly affects the isolator's ability to safeguard against user exposure to high voltage and is of paramount importance. The insulator is the "heart" of the isolation barrier and key to maintaining system safety. It is very important that the insulation be uniform, with no voids that can cause a localized breakdown. Insulator uniformity is a function of insulator material and the fabrication process.
The dielectric strength of the optocoupler's injection-molded plastic compound can vary by as much as 300% due to voids created during fabrication. In contrast, ISOpro isolators use semiconductor oxide layers for its primary insulator. The CMOS oxide deposition process is very tightly controlled and highly uniform, and the resulting variation in dielectric strength is only 20%.
Each oxide layer has a breakdown voltage of 500 VACRMS per micron (millionth of a meter). Higher voltages (e.g. 5 kVACRMS) are implemented by simply stacking oxide layers during wafer fabrication. The result is a higher absolute maximum breakdown voltage in a substantially smaller size compared to optocouplers and insulator reliability that is independent of the packaging process.
Safety Certifications
International safety standards provide test methodologies and guidelines to ensure endequipment safety from electrical shock, mechanical hazard, and fire and electromagnetic interference. Isolator classifications include "Basic" and "Reinforced". Basic isolation is intended to provide a single-level of protection against electrical shock only and cannot be considered failsafe. While these devices are accessible to a user, they must be contained within the system. Basic isolation devices are 100% safety tested at 2.5 kVACRMS for a period of 1 minute and, typically, have a minimum creepage of 3.2 mm. Reinforced isolation provides two levels of protection, making these devices failsafe and allowing user access. These devices are 100% safety tested at 5 kVACRMS for a period of 1 minute and typically provide a minimum creepage of 6.4 mm. Note that the exact creepage distance and rated isolation voltage requirements vary from one system to another according to the system's specifications and targeted environmental operating conditions (pollution degree). Certification testing involves subjecting the isolator component to various stresses as prescribed by test standards while monitoring the device under test for failures that could compromise safety.
